(1) Field of the Invention
The invention relates to the organic light-emitting diode devices, and more particularly to the organic light-emitting diode devices having reduced ambient-light reflection and methods for making the same.
(2) Description of the Prior Art
Owing to high brightness, fast response speed, light weight, thin and small features, full color, no viewing angle differences, no need for an LCD back-light board and low electrical consumption, an organic light emitting diode display or organic electroluminescence display (OLED) takes the lead to substitute a twist nematic (TN) or a super twist nematic (STN) liquid crystal display. Further, it substitutes for a small-sized thin-film transistor (TFT) LCD to become a new display material of fabricating portable information products, cell phones, personal digital assistant (PDA) and notebook.
Referring to FIG. 1, an organic light-emitting diode (OLED), also referred to as an organic electroluminescent (EL) device is constructed in a normal configuration on a light-transmissive substrate 10 through which the light emitted by the device is viewed, and a light-transmissive anode 12, an organic layer 14 are sequentially deposited thereon. The organic layer 14 comprises, in sequence, a hole-injecting layer (HIL), a hole-transporting layer (HTL), an emitting layer (EML) and an electron-transporting layer (ETL). Next a reflective metal cathode 16 is deposited thereon. When an electrical potential is placed across the electrodes, holes and electrons are injected into the organic zones from the anode and cathode, respectively. Light emission results from hole-electron recombination within the device and one part of light emitting through the light-transmissive substrate 10 and anode 12. And the other part of light will reflect by the reflective metal cathode 16 then also emit through the light-transmissive substrate 10 and anode 12. This kind of emission type is what we called “bottom emission.” On the contrary, if the light-transmissive anode 12 is deposited on a reflective substrate (or having a reflective metal layer on a light-transmissive substrate) then deposited same organic layer and a light-transmissive cathode that light will emit through the light-transmissive cathode. And this kind of emission type is so called “top emission.”
The foregoing of two emission types are all with the reflective metal electrode which is to improve brightness of emission from which internally generated light from the organic layer is reflected and directed toward the light-transmissive substrate. However, such a metallic electrode also reflects ambient-light entering the device structure through the light-transmissive substrate and the light-transmissive electrode, thereby degrading the visually perceived contrast of the emitted light, as viewed by an observer. Therefore, to avoid the ambient reflected light, a well known approach is depositing a “black cathode” (made by ZnO) which is a light absorptive and electric conductive material between the organic layer and the reflective cathode. And using the black cathode absorbs the ambient-light thereby reducing ambient-light reflection. Another well-known approach does not absorb the ambient-light. As shown in FIG. 2, a “black layer” 26 is deposited between the organic layer 22 and the metal cathode 28. By adjusting the thickness of the black layer 26, destructive optical interference of the reflected ambient-light is generated by the translucent electron-injecting cathode 24 and metal cathode 28, thereby reducing ambient-light reflection.
Either the black cathode or the black layer, the conductivity thereof is considerate. If materials are bad in conductivity, the applying voltage must increase. Further, the black layer comprises multi-layer, not only the conductivity but also the refractive index are the challenges to be dealt with. Additionally, one more layer we have, more time and cost we spend. Concerning about time to market, the OLED device that has less layers is more competitive. Thus, the present invention provides methods to reduce the ambient-light reflection without increasing layers in the OLED device.